1. Field of the Invention
The invention relates to an alkali-free aluminoborosilicate glass. The invention also relates to uses of this glass.
2. Background of the Invention
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal (or expressed differently: liquid crystal) display technology, for example in TN (twisted nematic)/STN (supertwisted nematic, or expressed differently: super twisted nematic) displays, active matrix liquid crystal displays (AMLCDs), thin-film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the process for the production of flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a low density. Use as substrate material for integrated semiconductor circuits, for example in TFT displays (“chip on glass”) in addition requires thermal matching to the thin-film material silicon which is usually deposited on the glass substrate in the form of amorphous silicon (a-Si) at low temperatures of up to 300° C. The amorphous silicon is partially recrystallized by subsequent heat treatment at temperatures of about 600° C. Owing to the a-Si fractions, the resulting, partially crystalline poly-Si layer is characterized by a thermal expansion coefficient of α20/300≅3.7×10−6/K. Depending on the a-Si/poly-Si ratio, the thermal expansion coefficient α20/300 may vary between 2.9×10−6/K and 4.2×10−6/K. When substantially crystalline Si layers are generated by high temperature treatments above 700° C. or direct deposition by CVD processes, which is likewise desired in thin-film photovoltaics, a substrate is required which has a significantly reduced thermal expansion of 3.2×10−6/K or less. In addition, applications in display and photovoltaics technology require the absence of alkali metal ions. Sodium oxide levels of less than 1000 ppm (parts per million) as a result of production can be tolerated in view of the generally “poisoning” action due to diffusion of Na+ into the semiconductor layer.
It should be possible to produce suitable glasses economically on a large industrial scale in adequate quality (no bubbles, knots, inclusions), for example in a float plant or by drawing methods. In particular, the production of thin (<1 mm) streak-free substrates with low surface undulation by drawing methods requires high devitrification stability of the glasses. In order to counter compaction of the substrate during production, in particular in the case of TFT displays, which has a disadvantageous effect on the semiconductor microstructure, the glass needs to have a suitable temperature-dependent viscosity characteristic line: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature, i.e. Tg>700° C., while on the other hand not having excessively high melting and processing (VA) temperature, i.e. a VA of <1350° C.
The requirements of glass substrates for LCD display technology or thin-film photovoltaics technology are also described in “Glass substrates for AMLCD applications: properties and implications” by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997), and in “Photovoltaik—Strom aus der Sonne” by J. Schmid, Verlag C. F. Muller, Heidelberg 1994, respectively.
The abovementioned requirement profile is fulfilled best by alkaline earth metal aluminoborosilicate glasses. However, the known display or solar cell substrate glasses described in the following publications still have disadvantages and do not meet the full list of requirements.
Some documents describe glasses containing relatively little or no BaO, e.g. European Patent Application No. 714 862 B1, International Patent Application No. 98/27019, Japanese Patent Application No. 10-72237 A, European Patent Application No. 510 544 B1, International Patent Application No. 98/11919 and International Patent Application No. 98/11920. Glasses of this type, in particular those having low coefficients of thermal expansion, i.e. low RO content and high network former content, are very susceptible to crystallization. Furthermore, most of the glasses, in particular in European Patent Application No. 714862 B1 and Japanese Patent Application No. 10-72237 A, have very high temperatures at a viscosity of 102 dPas.
However, the preparation of display glasses having high levels of the heavy alkaline earth metal oxides BaO and/or SrO is likewise associated with great difficulties owing to the poor meltability of the glasses. In addition, glasses of this type, as described, for example, in Federal Republic of Germany Patent Application No.37 30 410 A1, U.S. Pat. No. 5,116,789, U.S. Pat. No. 5,116,787, European Patent Application No.341 313 B1, Japanese Patent Application No. 9-169538 A, Japanese Patent Application No. 4-160030 A, European Patent Application No.510 543 B1 and Japanese Patent Application No. 9-100135 A, have an undesirably high density.
Glasses having relatively high levels of light alkaline earth metal oxides, in particular MgO, as described, for example, in Japanese Patent Application No. 9-156953 A, Japanese Patent Application No. 8-295530 A, Japanese Patent Application No. 9-48632 A and Federal Republic of Germany Patent Application No.197 39 912 C1, exhibit good meltability and have a low density. However, they do not meet all requirements made of display and solar cell substrates with regard to chemical resistance, in particular to buffered hydrofluoric acid, to crystallization stability and to heat resistance.
Glasses having low boric acid contents exhibit excessively high melting temperatures or, as a result of this, excessively high viscosities at the melt and processing temperatures required for processes involving these glasses. This applies to the glasses of Japanese Patent Application No. 10-45422 A and Japanese Patent Application No. 9-263421 A.
Moreover, glasses of this type have a high devitrification tendency when combined with low BaO contents.
In contrast, glasses having high boric acid contents, as described, for example, in U.S. Pat. No. 4,824,808, have insufficient heat resistance and chemical resistance, in particular to hydrochloric acid solutions.
Glasses having a relatively low SiO2 content do not have sufficiently high chemical resistance either, in particular when they contain relatively large amounts of B2O3 and/or MgO and are low in alkaline earth metals. This applies to the glasses of European Patent Application No.672 629 A2. The relatively SiO2-rich variants of the latter document have only low Al2O3 levels, which is disadvantageous for the crystallization behavior.
Federal Republic of Germany Patent No. 196 17 344 C1 (U.S. Pat. No. 5,908,703) and Federal Republic of Germany Patent No. 196 03 698 C1 (U.S. Pat. No. 5,770,535) by the Applicant disclose alkali-free, tin oxide-containing, low-BaO glasses having a coefficient of thermal expansion α20/300 of about 3.7×10−6 and very good chemical resistance. They are suitable for use in display technology. However, since they must contain ZnO, they are not ideal, in particular for processing in a float plant. In particular at higher ZnO contents (>1.5% by weight), there is a risk of formation of ZnO coatings on the glass surface by evaporation and subsequent condensation in the hot-shaping range.
The glasses described in Japanese Patent Application No. 9-12333 A for hard disks, are comparatively low in Al2O3 or B2O3, the latter merely being optional. The glasses have high alkaline earth metal oxide contents and have high thermal expansion, which makes them unsuitable for use in LCD or PV technology.
Federal Republic of Germany Patent No. 196 01 022 A1 describes SnO-containing glasses which are selected from a very wide composition range. The glasses, which, according to the examples, are rich in SnO, tend to exhibit glass defects because of the ZrO2 level which has to be present.
Federal Republic of Germany Patent Application No. 42 13 579 A1 describes glasses for TFT applications having a coefficient of thermal expansion α20/300 of <5.5×10−6/K, according to the examples of >4.0×10−6/K. These glasses which have relatively high B2O3 levels and relatively low SiO2 contents do not have a high chemical resistance, in particular to diluted hydrochloric acid.
U.S. Pat. No. 5,374,595 describes glasses having coefficients of thermal expansion of between 3.2×10−6/K and 4.6×10−6/K. The glasses which, as the examples illustrate, have high BaO content, are relatively heavy and exhibit poor meltability and a thermal expansion which is not ideally matched to substantially crystalline Si.
In the unexamined Japanese publications Japanese Patent Application No. 10-25132 A, Japanese Patent Application No. 10-114538 A, Japanese Patent Application No. 10-130034 A, Japanese Patent Application No. 10-59741 A, Japanese Patent Application No. 10-324526 A, Japanese Patent Application No. 11-43350 A, Japanese Patent Application No. 11-49520 A, Japanese Patent Application No. 10-231139 A and Japanese Patent Application No. 10-139467 A, mention is made of very wide composition ranges for display glasses, which can be varied by means of many optional components and which are admixed with one or more specific refining agents in each case. However, these documents do not indicate how glasses having the complete requirement profile described above can be obtained in a specific manner.